2Warm Up Solve each equation. 2.4 = x 9 2 1. 10.8 2. 1.6x = 1.8(24.8) 27.9Determine whether each data set could represent a linear function.3.x2468y123no4.x–2–11y–626yes
3ObjectiveSolve problems involving direct, inverse, joint, and combined variation.
4Vocabulary direct variation constant of variation joint variation inverse variationcombined variation
5In Chapter 2, you studied many types of linear functions In Chapter 2, you studied many types of linear functions. One special type of linear function is called direct variation. A direct variation is a relationship between two variables x and y that can be written in the form y = kx, where k ≠ 0. In this relationship, k is the constant of variation. For the equation y = kx, y varies directly as x.
6A direct variation equation is a linear equation in the form y = mx + b, where b = 0 and the constant of variation k is the slope. Because b = 0, the graph of a direct variation always passes through the origin.
7Example 1: Writing and Graphing Direct Variation Given: y varies directly as x, and y = 27 when x = 6. Write and graph the direct variation function.y = kxy varies directly as x.27 = k(6)Substitute 27 for y and 6 for x.k = 4.5Solve for the constant of variation k.y = 4.5xWrite the variation function by using the value of k.
8Example 1 ContinuedGraph the direct variation function.The y-intercept is 0, and the slope is 4.5.Check Substitute the original values of x and y into the equation.y = 4.5x(6)
9If k is positive in a direct variation, the value of y increases as the value of x increases. Helpful Hint
10Check It Out! Example 1Given: y varies directly as x, and y = 6.5 when x = 13. Write and graph the direct variation function.y = kxy varies directly as x.6.5 = k(13)Substitute 6.5 for y and 13 for x.k = 0.5Solve for the constant of variation k.y = 0.5xWrite the variation function by using the value of k.
11Check It Out! Example 1 Continued Graph the direct variation function.The y-intercept is 0, and the slope is 0.5.Check Substitute the original values of x and y into the equation.y = 0.5x(13)
12When you want to find specific values in a direct variation problem, you can solve for k and then use substitution or you can use the proportion derived below.
13Example 2: Solving Direct Variation Problems The cost of an item in euros e varies directly as the cost of the item in dollars d, and e = 3.85 euros when d = $5.00. Find d when e = euros.Method 1 Find k.e = kd3.85 = k(5.00)Substitute.0.77 = kSolve for k.Write the variation function.e = 0.77dUse 0.77 for k.10.00 = 0.77dSubstitute for e.12.99 ≈ dSolve for d.
14Example 2 ContinuedMethod 2 Use a proportion.e1=d1d2e23.85=5.00d10.00Substitute.3.85d = 50.00Find the cross products.12.99 ≈ dSolve for d.
15Check It Out! Example 2The perimeter P of a regular dodecagon varies directly as the side length s, and P = 18 in. when s = 1.5 in. Find s when P = 75 in.Method 1 Find k.P = ks18 = k(1.5)Substitute.12 = kSolve for k.Write the variation function.P = 12sUse 12 for k.75 = 12sSubstitute 75 for P.6.25 ≈ sSolve for s.
16Check It Out! Example 2 Continued Method 2 Use a proportion.P1=s1s2P218=1.5s75Substitute.18s = 112.5Find the cross products.6.25 = sSolve for s.
17A joint variation is a relationship among three variables that can be written in the form y = kxz, where k is the constant of variation. For the equation y = kxz, y varies jointly as x and z.The phrases “y varies directly as x” and “y is directly proportional to x” have the same meaning.Reading Math
18Example 3: Solving Joint Variation Problems The volume V of a cone varies jointly as the area of the base B and the height h, and V = 12 ft3 when B = 9 ft3 and h = 4 ft. Find b when V = 24 ft3 and h = 9 ft.Step 1 Find k.Step 2 Use the variation function.V = kBh1312 = k(9)(4)Substitute.V = BhUse for k.1313= kSolve for k.1324 = B(9)Substitute.8 = BSolve for B.The base is 8 ft2.
19Check It Out! Example 3The lateral surface area L of a cone varies jointly as the area of the base radius r and the slant height l, and L = 63 m2 when r = 3.5 m and l = 18 m. Find r to the nearest tenth when L = 8 m2 and l = 5 m.Step 1 Find k.Step 2 Use the variation function.L = krl63 = k(3.5)(18)Substitute.L = rlUse for k. = kSolve for k.8 = r(5)Substitute.1.6 = rSolve for r.
20A third type of variation describes a situation in which one quantity increases and the other decreases. For example, the table shows that the time needed to drive 600 miles decreases as speed increases.This type of variation is an inverse variation. An inverse variation is a relationship between two variables x and y that can be written in the form y = , where k ≠ 0. For the equation y = , y varies inversely as x.kx
21Example 4: Writing and Graphing Inverse Variation Given: y varies inversely as x, and y = 4 when x = 5. Write and graph the inverse variation function.kxy =y varies inversely as x.k5Substitute 4 for y and 5 for x.4 =k = 20Solve for k.20xWrite the variation formula.y =
22x y –2 –10 –4 –5 –6 – –8 x y 2 10 4 5 6 8 Example 4 Continued To graph, make a table of values for both positive and negative values of x. Plot the points, and connect them with two smooth curves. Because division by 0 is undefined, the function is undefined when x = 0.xy–2–10–4–5–6––8xy21045681031035252
23Check It Out! Example 4Given: y varies inversely as x, and y = 4 when x = 10. Write and graph the inverse variation function.kxy =y varies inversely as x.k10Substitute 4 for y and 10 for x.4 =k = 40Solve for k.40xWrite the variation formula.y =
24Check It Out! Example 4 Continued To graph, make a table of values for both positive and negative values of x. Plot the points, and connect them with two smooth curves. Because division by 0 is undefined, the function is undefined when x = 0.xy–2–20–4–10–6––8–5xy220410685203203
25When you want to find specific values in an inverse variation problem, you can solve for k and then use substitution or you can use the equation derived below.
26Example 5: Sports Application The time t needed to complete a certain race varies inversely as the runner’s average speed s. If a runner with an average speed of 8.82 mi/h completes the race in 2.97 h, what is the average speed of a runner who completes the race in 3.5 h?kst =Method 1 Find k.k8.822.97 =Substitute.k =Solve for k.st =Use for k.sSubstitute 3.5 for t.3.5 =Solve for s.s ≈ 7.48
27Example 5 ContinuedMethod Use t1s1 = t2s2.t1s1 = t2s2(2.97)(8.82) = 3.5sSubstitute.= 3.5sSimplify.7.48 ≈ sSolve for s.So the average speed of a runner who completes the race in 3.5 h is approximately 7.48 mi/h.
28Check It Out! Example 5The time t that it takes for a group of volunteers to construct a house varies inversely as the number of volunteers v. If 20 volunteers can build a house in 62.5 working hours, how many working hours would it take 15 volunteers to build a house?Method 1 Find k.k2062.5 =Substitute.kvt =k = 1250Solve for k.1250vt =Use 1250 for k.125015Substitute 15 for v.t =13t ≈ 83Solve for t.
29Check It Out! Example 5 Continued Method 2 Use t1v1 = t2v2.t1v1 = t2v2(62.5)(20) = 15tSubstitute.1250 = 15tSimplify.1383 ≈ tSolve for t.So the number of working hours it would take 15 volunteers to build a house is approximately hours.1383
30You can use algebra to rewrite variation functions in terms of k. Notice that in direct variation, the ratio of the two quantities is constant. In inverse variation, the product of the two quantities is constant.
31Example 6: Identifying Direct and Inverse Variation Determine whether each data set represents a direct variation, an inverse variation, or neither.A.In each case xy = 52. The product is constant, so this represents an inverse variation.x6.513104y840.5B.yxx5812y304872In each case = 6. The ratio is constant, so this represents a direct variation.
32Example 6: Identifying Direct and Inverse Variation Determine whether each data set represents a direct variation, an inverse variation, or neither.C.x368y51421Since xy and are not constant, this is neither a direct variation nor an inverse variation.yx
33x 3.75 15 5 y 12 3 9 x 1 40 26 y 0.2 8 5.2 Check It Out! Example 6 Determine whether each data set represents a direct variation, an inverse variation, or neither.6a.x3.75155y1239In each case xy = 45. The ratio is constant, so this represents an inverse variation.6b.x14026y0.285.2In each case = 0.2. The ratio is constant, so this represents a direct variation.yx
34A combined variation is a relationship that contains both direct and inverse variation. Quantities that vary directly appear in the numerator, and quantities that vary inversely appear in the denominator.
35Example 7: Chemistry Application The change in temperature of an aluminum wire varies inversely as its mass m and directly as the amount of heat energy E transferred. The temperature of an aluminum wire with a mass of 0.1 kg rises 5°C when 450 joules (J) of heat energy are transferred to it. How much heat energy must be transferred to an aluminum wire with a mass of 0.2 kg raise its temperature 20°C?
36Step 2 Use the variation function. Example 7 ContinuedStep 1 Find k.Step 2 Use the variation function.kEmCombined variationΔT =E900mΔT =Use for k.1900k(450)0.15 =Substitute.E900(0.2)20 =Substitute.1900= kSolve for k.3600 = ESolve for E.The amount of heat energy that must be transferred is 3600 joules (J).
37Check It Out! Example 7The volume V of a gas varies inversely as the pressure P and directly as the temperature T. A certain gas has a volume of 10 liters (L), a temperature of 300 kelvins (K), and a pressure of 1.5 atmospheres (atm). If the gas is heated to 400K, and has a pressure of 1 atm, what is its volume?
38Check It Out! Example 7Step 1 Find k.Step 2 Use the variation function.kTPCombined variationV =0.05TPUse 0.05 for k.V =k(300)1.510 =Substitute.0.05(400)(1)V =Substitute.0.05 = kSolve for k.V = 20Solve for V.The new volume will be 20 L.
39Lesson Quiz: Part I1. The volume V of a pyramid varies jointly as the area of the base B and the height h, and V = 24 ft3 when B = 12 ft2 and h = 6 ft. Find B when V = 54 ft3 and h = 9 ft.18 ft22. The cost per person c of chartering a tour bus varies inversely as the number of passengers n. If it costs $22.50 per person to charter a bus for 20 passengers, how much will it cost per person to charter a bus for 36 passengers?$12.50
40Lesson Quiz: Part II3. The pressure P of a gas varies inversely as its volume V and directly as the temperature T. A certain gas has a pressure of 2.7 atm, a volume of 3.6 L, and a temperature of 324 K. If the volume of the gas is kept constant and the temperature is increased to 396 K, what will the new pressure be?3.3 atm